The UNIX Memory API

University of New Mexico Memory Virtualization: The UNIX Memory API Prof. Patrick G. Bridges 1 University of New Mexico Memory API: malloc() #include <stdlib.h> void* malloc(size_t size) Main programmer-level API in C. Language-level interface, not the actual OS interface Allocate a memory region on the heap. ▪ Argument ▪ size_t size : size of the memory block(in bytes) ▪ size_t is an unsigned integer type. ▪ Return ▪ Success : a void type pointer to the memory block allocated by malloc ▪ Fail : a null pointer free/calloc/realloc also exist in the same vein 2 University of New Mexico Lots of ways to go wrong with memory Some sample things we’ve all done ▪ Not copying enough data (e.g. terminating nulls) ▪ Not allocating space for the data copied ▪ Not freeing data (memory leaks) ▪ Accessing data after its freed ▪ Freeing an area multiple times What does each of these actually do? Requires understanding how the language API is built on top of the OS memory API 3 University of New Mexico Memory Management System Calls #include <unistd.h> int brk(void *addr) void *sbrk(intptr_t increment); malloc malloc library call use brk and/or mmap system calls. ▪ brk is called to expand the program’s break. ▪ break: The location of the end of the heap in address space ▪ sbrk is an additional call similar with brk. ▪ Programmers should never directly call either brk or sbrk. What does this actually do? 4 University of New Mexico How do brk and sbrk work? Address Space The greyed-out area of an address 0x400000 Code space is not actually allocated. (Text) 0x401000 To use it, the OS has to make it Data 0xcf2000 available. Heap 0xd13000 Brk/sbrk sets the location of the boundary between allocated heap heap memory and unallocated memory! (free) stack 0x7fff9ca28000 Stack 0x7fff9ca49000 5 University of New Mexico System Calls(Cont.) #include <sys/mman.h> void *mmap(void *ptr, size_t length, int port, int flags, int fd, off_t offset) ▪ mmap system call can create an anonymous memory region. 6 University of New Mexico What about mmap? Address Space mmap lets the program request finer- 0x400000 Code grain allocation of parts of its address (Text) 0x401000 space Data 0xcf2000 More than just moving the program Heap break 0xd13000 Note that the address space is now heap disjoint! mmap region mmap can also do implicit file I/O – (free) that’s a later topic… stack 0x7fff9ca28000 Stack 0x7fff9ca49000 7 University of New Mexico How are malloc/new implemented? The language runtime (libc) gets memory in chunks from the operating system ▪ Using mmap or sbrk – 4k or more at a time ▪ (Why not in smaller pieces?) The language runtime divides these big blocks up to satisfy malloc/new requests ▪ Basic data structure is a “free list”, a linked list of free chunks of memory ▪ Malloc/new searches list to find a chunk to satisfy an allocation request ▪ Free returns things to this list Important questions ▪ Where are the pointers for the linked lists stored? ▪ What block do you use to satisfy an allocation request? 8 University of New Mexico Fragmentation: Storage Virtualization Enemy #1 We generally have to divide big blocks into smaller blocks, and there’s rarely an exact fit ▪ Variable-size allocations can result in with small, hard-to-allocate blocks ▪ Sometimes have to allocate space bigger than we want to Wasted space from storage allocation is called fragmentation ▪ Internal fragmentation – wasted space that’s allocated but not used. (You want 5 bytes but we have to allocate 8) ▪ External fragmentation – small bits we can’t allocate How do we allocate storage to handle fragmentation? 9.

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